1. Field of the Invention
The present invention relates to an exposure processing method of a planographic printing plate of thermal recording type, which printing plate contains a compound encapsulating microcapsule in an image recording layer provided on a support body so as to be capable of being exposure processed, and an apparatus for executing the same method.
2. Description of the Related Art
For offset printing, generally, a photosensitive plate (PS plate) is used. In the field of planographic printing, a planographic printing plate has been proposed for use in computer-to-plate (CTP) systems for making printing plates directly by laser exposure processing on the basis of digital data of computer or the like.
In such CTP systems, exposing planographic printing plates and then mounting them directly on a printing press for printing, without a development process, has been proposed before. This is known as on-press development. That is, CTP systems are methods of using a recording layer (photosensitive layer) in which removing non-image portions of a printing plate precursor to form a printing plate can occur in ordinary printing process, such that development is effected on a printing press after exposing, to swiftly obtain a printing plate of a finished state, without requiring passing the plate to a specific developing process (that is, on-press development systems).
In these on-press development systems, aluminum plates having a hydrophilic surface anodized and surface roughened are used as support bodies. On this hydrophilic surface, a recording layer (photosensitive layer) is formed containing hydrophobic thermoplastic polymer particles, that can be coalesced under the effect of heat but normally dispersed in a hydrophilic binder. A compound for converting light into heat is contained in this recording layer or its adjacent layer. That is, a printing plate of thermal recording type (heat mode) is used as a planographic printing plate.
In on-press development Systems, exposure of planographic printing plates as printing plates of thermal recording type (heat mode) is executed by using, for example, laser beam (LED, light emitting diode) of a wavelength range in the infrared (IR) ray or near infrared ray regions.
In exposuring a planographic printing plate, which is a printing plate of thermal recording type (heat mode), the laser beam emitted onto the recording layer corresponding to the desired image is converted to heat. Hydrophobic thermoplastic polymer particles contained in the recording layer are heated equal to or above their solidifying temperature, to be solidified to grow hydrophobic lumps in the hydrophilic layer, and are made insoluble to ordinary water or aqueous solutions so as not to drop out and disperse.
In this on-press development system, the exposed planographic printing plate is mounted, for example, on a printing cylinder of a printing press. By stating the printing press, a dampening roller to supply aqueous dampening solution is rotated in contact with the recording layer of the planographic printing plate, and an ink roller is also rotated in contact with the recording layer of the planographic printing plate, and the printing process is carried out. As a result, solidified (cured) hydrophobic lumps are left remaining on the recording layer of the planographic printing plate, and other hydrophilic layer parts are removed by being washed off and dispersed in ordinary water or an aqueous solution, whereby the plate is developed.
Usually, after about 10 times of rotation of printing cylinder, a first clean and useable print is obtained (see, for example, the specification of Japanese Patent No. 2938397).
In a conventional planographic printing plate for on-press development (planographic printing plate which forms an image by a portion thereof being washed off and dispersed by using water or dampening water after the portion is exposed), there is a composition proposed containing a microcapsule encapsulating at least an ethylenically unsaturated compound and a polymerization initiator as active substances, in an image recording layer provided on a support body of a planographic printing plate. Active substances diffusing from the microcapsule are present on the surface of the microcapsule and near the surface.
In this case, when the image recording layer is exposed by an active beam, the film of the image recording layer is cured, and a continuous firm film is formed, and the film of the unexposed portions is washed off by water or aqueous solution (that is, the film of unexposed portion is removed by being washed off and dispensed by water or aqueous solution). (See, for example, Japanese Patent Publication No. 2639748.)
In such a conventional on-press development system, as shown in FIG. 12B and FIG. 12C, an aluminum support body 11 is used as a support body of a planographic printing plate 17 which is a printing plate of thermal recording type (heat mode). When it is exposed by a laser beam, the laser beam is emitted within a high output range so as not to cause ablation (a phenomenon of burning out the illuminated portion due to the intense energy), near and above, in the thickness direction, the layer which is the recording layer 15 of a planographic printing plate 17, but heating to sufficiently above the solidifying temperature.
In the recording layer 15 of the planographic printing place 17 as a printing plate of conventional thermal recording type (heat mode), the entire recording layer 15 is formed uniformly of a material having a specified light absorption coefficient.
Accordingly, when the planographic printing plate 17 of thermal recording type (heat mode) is exposed by laser beam, as shown in FIG. 12A, the light energy absorbed in the recording layer 15 is larger at the upside (surface) in the thickness direction of the recording layer 15 receiving the laser beam, and smaller at the downside (aluminum support body side) in thickness direction of the recording layer 15.
As a result, as shown in FIG. 12C, it is hard to heat sufficiently near the downside in thickness direction of the film in the recording layer 15. In other words, it is hard to raise the sensitivity of the planographic printing plate 17 to allow sufficient heating of the near-support portion of the film in the recording layer 15.
Further, when the sensitivity of the planographic printing plate 17 is low, the heat energy converted from the emitted laser beam near the downside in thickness direction of the film in the recording layer 15 (near the hydrophilic surface 13 of anodic oxide film) rapidly diffuses and escapes to the aluminum support body 11 side of high thermal conductivity. Hence, if emitting the laser beam at a maximum output so as not to cause ablation, the temperature is not raised sufficiently above the solidifying temperature near the interface of the hydrophilic surface 13 of the aluminum support body 11 and the recording layer 15 (heating recording layer).
Therefore, curing of the portions of the recording layer 15 illuminated with laser beam (image portions) are not promoted sufficiently, and the strength of the image portions may be insufficient.
In such a planographic printing plate of low sensitivity, when a dampening solution is applied on to the recording layer 15 for printing, since curing by the exposure process is not sufficiently progressed near the downside in the thickness direction of the layer (the interface) in the recording layer 15, portions of insufficient strength are washed off and diffuse in water or aqueous solution in the development process. As a result, as the number of prints increases, the image exposed and formed on the recording layer 15 may be easily lost and print resistance is lowered.